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<!DOCTYPE HTML>
<html>
<head>
    <style>
        body {
            margin: 0px;
            padding: 0px;
        }
​
        #myCanvas {
            border: 1px solid #9C9898;
        }
    </style>
    <script>
       var mousePos, previousMousePos;
      
        function writeMessage(canvas, message) {
            var context = canvas.getContext('2d');
            context.clearRect(0, 0, canvas.width, canvas.height);
            context.font = '18pt Calibri';
            context.fillStyle = 'black';
            context.fillText(message, 10, 25);
        }
​
        function getMousePos(canvas, evt) {
            // get canvas position
            var obj = canvas;
            var top = 0;
            var left = 0;
            while (obj && obj.tagName != 'BODY') {
                top += obj.offsetTop;
                left += obj.offsetLeft;
                obj = obj.offsetParent;
            }
​
            // return relative mouse position
            var mouseX = evt.clientX - left + window.pageXOffset;
            var mouseY = evt.clientY - top + window.pageYOffset;
            return {
                x:mouseX,
                y:mouseY
            };
        }
​
        window.onload = function () {
            var canvas = document.getElementById('myCanvas');
            var context = canvas.getContext('2d');
            var started = false, previousMousePos;
​
          canvas.addEventListener('mousedown', clicked);
          canvas.addEventListener('mouseup', released);
          
          function clicked(evt) {
            previousMousePos = getMousePos(canvas, evt);
            started = true;
          }
          
           function released(evt) {
            started = false;
          }
          
            canvas.addEventListener('mousemove', function (evt) {
               mousePos = getMousePos(canvas, evt);
                var message = "Mouse position: " + mousePos.x + "," + mousePos.y;
                // TRY TO UNCOMMENT THIS LINE !
                //writeMessage(canvas, message);
​
                // Let's draw some lines that follow the mouse pos
                if (started) {
                    context.beginPath();
                    context.moveTo(previousMousePos.x, previousMousePos.y);
                    context.lineTo(mousePos.x, mousePos.y);
                    context.stroke();
​
                    previousMousePos = mousePos;
                }
            }, false);
        };
    </script>
</head>
<body onmousedown="return false;">
<canvas id="myCanvas" width="578" height="400">
</canvas>
</body>
</html>
 
if (document.getElementById('hello')) {
  document.getElementById('hello').innerHTML = 'Hello World - this was inserted using JavaScript';
}
​
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